This application claims the benefit of Korean Patent Application No. 10-2001-0005973, filed on Feb. 7, 2001, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a reflective liquid crystal display (LCD) device, and more particularly, to a reflective LCD device using a cholesteric liquid crystal color filter.
2. Description of the Related Art
Until recently, the cathode-ray tube (CRT) has been mainly used for display systems. However, flat panel displays are beginning to be implemented for display systems because of their small depth dimensions, desirably low weight, and low voltage power supply requirements. Currently, thin film transistor-liquid crystal displays (TFT-LCDs) with high resolution and small depth dimension are being developed.
Generally, conventional thin film transistor liquid crystal display (TFT-LCD) devices include an upper substrate and a lower substrate with a liquid crystal molecule layer interposed therebetween. The upper substrate and the lower substrate are generally referred to as a color filter substrate and an array substrate, respectively.
LCD devices use backlight sources disposed under the lower substrate to provide light. However, only about 7% of the light that is emitted by the backlight passes through each cell of the LCD devices. Since the backlight should emit light of a relatively high brightness, corresponding power consumption increases. Accordingly, a large capacity heavy battery is commonly used to supply sufficient power for the backlight. Moreover, use of the large capacity battery limits operating time.
Since power consumption of reflective LCD devices greatly decreases due to use of ambient light as a light source which increases operating time, reflective LCD devices are used for portable information apparatuses such as electric diaries and personal digital assistants (PDAs). In reflective LCD devices, a pixel area, which is covered with a transparent electrode in conventional transmissive LCD devices, is covered with a reflective plate or reflective electrode having opaque reflection characteristics. However, brightness of reflective LCD devices is very poor because the devices use only ambient light as a light source. The poor brightness results from operational characteristics of the reflective LCD devices, in which ambient light passes through a color filter substrate, is reflected on a reflective electrode on a lower substrate, passes through the color filter substrate again and then displays an image. Accordingly, brightness is decreased as a result of reduction of the transmittance when the ambient light passes through the color filter substrate twice. Since overall thickness of the color filter substrate is inversely proportional to transmittance and is directly proportional to color purity of the light, the problem of inadequate brightness of the reflective LCD devices can be remedied by forming a thin color filter with high transmittance and low color purity. However, there is a limit in fabricating the color filter below a threshold thickness due to characteristics of the resin used to form the color filter. Accordingly, one possible solution to this problem is forming LCD devices using cholesteric liquid crystals (CLCs) having selective reflection and transparency characteristics.
In reflective LCD devices using a CLC color filter layer, the fabrication processes are simplified due to omission of the reflective layer, and a high color purity and high contrast ratio is achieved. Moreover, since CLCs have a spiral structure and spiral pitch determines a selective reflection bandwidth of the CLCs, the reflection bandwidth can be controlled by a distribution of the spiral pitch at one pixel. To illustrate this in more detail, a wavelength range of visible light is from about 400 nm to 700 nm. The wavelength of the red light region is centered at about 650 nm, the wavelength of the green light region is centered at about 550 nm, and the wavelength of the blue light region is centered at about 450 nm. The CLC color filter is formed having characteristics that can selectively reflect or transmit right-handed or left-handed circularly polarized light at a bandwidth that corresponds to a pitch deviation by selecting bandwidths corresponding to the red, green, and blue light regions. In addition, the CLC color filter is formed having characteristics that control conditions for right or left pitch deviations with respect to the center wavelength. Accordingly, the pitch of the liquid crystal can be artificially adjusted so that a CLC color filter can selectively reflect light of an intrinsic wavelength of the color corresponding to each pixel.
FIG. 1 is a schematic cross-sectional view of a conventional reflective LCD device using a CLC color filter. The LCD device is an active matrix liquid crystal display (AMLCD) in which one TFT, which is an on/off switching device of a pixel voltage, controls a voltage of the liquid crystal of one pixel and adjusts transmittance of the pixel. In FIG. 1, upper and lower substrates 10 and 30 face each other and are spaced apart with a liquid crystal layer 50 interposed therebetween. At the bottom of the transparent substrate 1 of the upper substrate, a TFT xe2x80x9cTxe2x80x9d is formed. Beneath the TFT xe2x80x9cT,xe2x80x9d a pixel electrode 16 connected to the TFT xe2x80x9cTxe2x80x9d is formed in each pixel area. Beneath the pixel electrode 16, a black matrix 14 that screens light of non-operating areas of the liquid crystal is formed. At the top of the transparent substrate 1, a quarter wave plate (QWP) 18 and a polarization plate 20 are sequentially layered. On the transparent substrate 1 of the lower substrate 30, the CLC color filter 32, which reflects light of the bandwidth corresponding to the red, green, and blue regions and transmits light of other bandwidths, is formed. Beneath the CLC color filter 32, a light absorption layer 34 that absorbs the transmitted light through the CLC color filter 32 is formed. Upon the CLC color filter 32, a common electrode 36 is formed for applying an electric field to the liquid crystal layer 50 along with the pixel electrode 16. Reflective LCD devices using the conventional CLC color filter include the TFT, pixel electrode, and black matrix formed on the upper substrate with the CLC color filter and light absorption layer being formed on the lower substrate.
The following descriptions demonstrate relationships of aperture ratio of a pixel electrode of a reflective LCD device. The aperture ratio is related to brightness and a ratio of effective pixel area to total display area. The higher the aperture ratio, the higher the brightness. Furthermore, since the aperture ratio of reflective LCD devices is related to the brightness of the reflected light that determines characteristics of the devices, it is necessary to increase the aperture ratio of the reflective LCD devices.
FIG. 2 is a schematic plane view of the upper substrate of FIG. 1. In FIG. 1, a matrix structure is formed by orthogonal gate and data lines 11 and 13. A pixel electrode 16 is formed in a pixel area which is defined by the intersections of the gate line 11 and data line 13. A TFT xe2x80x9cTxe2x80x9d is formed to be connected to the pixel electrode 16 and a black matrix 14 is formed at an oblique-lined region. Since the pixel electrode 16 is formed separately from the gate line 11 and data line 16 in order to avoid any electrical interference therebetween, the aperture ratio is less than about 80%. Moreover, the black matrix 14 is formed overlapping an edge portion of the pixel electrode 16 to prevent light leakage at the edge of the pixel electrode caused by any cross-talk generated between the pixel electrode 16 and the data line 14, thereby deteriorating display quality. Consequently, since a total area of the pixel electrode 16 is not used as an operating area of the display in the reflective LCD device, this causes the deterioration in the brightness of the reflected light.
Accordingly, the present invention is directed to a reflective liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to improve a display quality of a reflective LCD device.
Another object of the present invention is to increase a pixel area of a reflective LCD device.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a reflective liquid crystal display device includes an upper substrate, a lower substrate, a liquid crystal layer interposed between the upper and lower substrates, a common electrode beneath the upper substrate, a plurality of orthogonal gate lines and data lines disposed on the lower substrate, a plurality of thin film transistors, each of the thin film transistors disposed adjacent to a crossing region of the gate and data lines and include source and drain electrodes, a light absorption layer formed on each of the plurality of thin film transistors, a cholesteric liquid crystal color filter layer formed on the light absorption layer, and a plurality of pixel electrodes formed on the cholesteric liquid crystal color filter layer.
In another aspect, a method of fabricating a reflective liquid crystal display device includes steps of forming upper and lower substrates, forming a liquid crystal layer interposed between the upper and lower substrates, forming a common electrode beneath the upper substrate, forming a plurality of orthogonal gate and data lines on the lower substrate, forming a plurality of thin film transistors, each thin film transistor disposed adjacent to a crossing region of the plurality of gate and data lines and include source and drain electrodes, forming a light absorption layer on the thin film transistors, forming a cholesteric liquid crystal color filter layer on the light absorption layer, and forming a plurality of pixel electrodes on the cholesteric liquid crystal color filter layer.
In another aspect, a reflective liquid crystal display device includes an upper substrate, a lower substrate, a liquid crystal layer interposed between the upper and lower substrates, a common electrode beneath the upper substrate, a plurality of orthogonal gate lines and data lines disposed on the lower substrate, a light absorption layer formed on the plurality of orthogonal gate and data lines, a plurality of thin film transistors located adjacent a crossing region of the gate and data lines and embedded within the light absorption layer, each of the plurality of thin film transistors include source and drain electrodes, a cholesteric liquid crystal color filter layer formed on the light absorption layer, and a plurality of pixel electrodes formed on the cholesteric liquid crystal color filter layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.